Heat storage device

ABSTRACT

A heat storage device comprises a heat storage tank  2  charged with a heat storage material  1  for storing the heat supplied from the outside, and a heat exchanger  3  for executing an injection and an extraction of heat between the inside of the storage tank  2  and the outside by the heat exchange between the heat storage material and a heat transfer medium. The heat exchanger  3  is disposed so as to execute a heat exchange between the central portion  2   a  in the heat storage tank  2  and the outside, and suppresses the natural convection of the heat storage material  1  of the outer portion  2   b  by, for example, dispersing a liquid-absorbent material  5  in the outer portion  2   b  surrounding the central portion in the heat storage tank  2 , whereby reduces the influence of external environment on the central portion  2   a  in the heat storage tank, thereby suppressing the heat loss toward the outside.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of Application of Ser. No.09/505,687, filed on Feb. 17, 2000, now U.S. Pat. No. 6,371,198.

FIELD OF THE INVENTION

The present invention relates to a heat storage device for temporarilystoring heat.

DESCRIPTION OF THE RELATED ART

Basic constructions of conventional heat storage devices are illustratedin FIGS. 11 through 13 in the form of sectional views.

A heat storage device shown in FIG. 11 comprises a heat storage tank 32charged with a heat storage material 31 such as water, and a heatexchanger 33 disposed in the heat storage tank 32, for executing a heatexchange with the outside. In order to store the heat of, for example, atemperature T_(x) within this heat storage device, the temperature ofthe heat storage material 31 is raised to T_(x) by adding the heat of atemperature of at least T_(x) to the heat exchanger 33 from the outside.When heat is needed at the outside, the heat of the temperature T_(x) isextracted from the heat storage material 31 to the outside, using theheat exchanger 33.

A heat storage device as shown in FIG. 12 comprises heat storage tanks42 a and 42 b charged with a heat transfer medium 47 such as water whichalso serves as a heat storage material for storing heat, wherein theheat storage tank 42 a has a communicating tube 44 a for connecting theheat storage tank 1 with the outside, wherein the heat storage tanks 42a and 42 b are connected by a communicating tube 44 b, and wherein theheat storage tank 42 b has a communicating tube 44 c for connecting theheat storage tank 42 b with the outside.

In the operation of this heat storage device, in order to store the heatof, for example, a temperature T_(x), the heat transfer medium 47 issucked out from the communicating tube 44 c, and the temperature of theheat transfer medium 47 is raised to at least T_(x) by adding heat tothe heat transfer medium 47 at the outside, then the heat transfermedium 47 being injected from the communicating tube 44 a to the heatstorage tanks 42 a. The heat transfer medium 47 removed from the heatstorage tank 42 a by this injection, arrives at the heat storage tanks42 a passing through the communicating tube 44 a. After a while, theheat transfer medium 47 is sucked out from the communicating tube 44 c,and executes a heat transport between the heat storage tank 42 and theoutside, then returning to the heat storage tank 42 a again passingthrough the communicating tube 44 a. Thus, a circulation of the heattransfer medium 47 is accomplished. If heat is needed at the outside, aprocess in the opposite direction to the injection process of heat isexecuted. That is, the heat transfer medium 47 is sucked out from thecommunicating tube 44 a, and heat is absorbed and utilized at theoutside, and the heat transfer medium 47 of which temperature hasdecreased as a result of the absorption and utilization of heat, isreturned from the communicating tube 44 c to the heat storage tank 41 b.Due to this returning, the heat transfer medium 47 removed from the heatstorage tanks 42 b flows into the heat storage tank 42 a passing throughthe communicating tube 44 b. Thus, a circulation of the heat transfermedium 47 is accomplished.

An object of suitably dividing the heat storage tank is to suppress theoccurrence of a dead water region where flow is apt to stagnate withinthe heat storage tank, and to allow the circulation of the heat transfermedium 47 in injecting and extracting heat to be uniformly conducted byusing the whole of the heat storage tank.

If the heat of a lower temperature than an ambient temperature isstored, the directions of flow of the heat transfer medium 47 in theprocesses of injection and extraction will be opposite to each other.However, the basic operation is similar to the foregoing.

On the other hand, in a heat storage device shown in FIG. 13, a heatstorage material 51 utilizing mainly transition heat is charged intosmall vessels 56, which are accommodated in the heat storage tank 52,and the heat storage tank 52 is provided with communicating tubes 55 aand 55 b for making the heat storage tank 52 communicate with theoutside. By the heat transfer medium 57 which circulates through thesecommunicating tubes 55 a and 55 b and the heat storage tank, heatexchange means for executing a heat exchange with the outside isconstituted. In this case, the storage of heat is mainly executed by theheat storage material 51 which utilizes a transition heat. Although theheat transfer medium 57 functions as heat transport means, it does notmean that the heat transfer medium 57 does not participate in thestorage of heat.

In this heat storage device, in order to store the heat of, for example,a temperature T_(x), the heat transfer medium 57 having a temperature ofat least T_(x) is injected from the outside to the heat storage tank 52through the communicating tube 55 a. The heat storage material 51 isheated by the heat which has been released by the heat transfer medium57 via the walls of the small vessels 56. The heat transfer medium 57injected returns to the outside passing through the communicating tube55 b, and is again heated to a temperature of at lease T_(x). Thus, asimilar circulation is repeated. If heat is needed at the outside, aprocess in the opposite direction to the process of heat injection isexecuted. That is, the heat transfer medium 57 is injected from theoutside to the heat storage tank 52 through the communicating tube 55 b.The heat transfer medium 57 injected is heated by the heat released bythe heat transfer medium 51, via the walls of the small vessels 56. Theheat transfer medium 57 heated returns to the outside passing throughthe communicating tube 55 a, and its temperature becomes lower thanT_(x) as a result of the utilization at the outside, then being injectedagain. Thus, a similar circulation is repeated. Also in the case wherethe heat of a lower temperature than the ambient temperature is stored,the basic operation is similar to the foregoing.

In this example of constitution, since the heat transfer medium 57 whichis injected and extracted from the outside passes through the gapsbetween the small vessels, the area of a heat transfer surface to theheat storage material 51 becomes large, as well as the treatment of theheat storage material 51 becomes easy, and hence such a constitution isoften used for heat storage devices that use paraffin as a heat storagematerial 51 which utilizes transition heat.

In the above-described heat storage devices shown in FIGS. 11 through13, in the state of heat storage, if there are temperature differencesbetween the heat storage materials 31, 51, and the heat transfer medium47 which also serves as a heat storage material, and the externalenvironment surrounding the heat storage tanks 32, 42, and 52, then heattransfer always takes place between the heat storage devices and theexternal environment through the wall surfaces of heat storage tanks 32,42, and 52, respectively.

Neglecting the influence of heat radiation which is usually low, theheat loss Q of a heat storage material or a heat transfer medium whichalso serves as a heat storage material (hereinafter these are bothreferred to as a heat storage material) is expressed by the followingequation.

Q=kA(Tx′−T ₀)dt [J]  (1)

Here, k denotes an overall heat transfer coefficient determined by thematerial, construction, and ambient air speed of a heat storage tank,etc. A denotes a contact area between a heat storage tank and a ambientfluid (such as air). T₀ represents an environmental temperature outsidethe heat storage device, and T_(x)′ represents a temperature at thesurface where the heat storage material contacts the heat storage tank(the surface of heat storage material), t representing an elapsed time.Since k and A can usually be regarded as constants irrespective of time,the equation (1) may be expressed approximately as follows:

Q=kA(Tx′−T ₀)dt [J]  (2)

Some heat storage methods mainly utilize a sensible heat of the heatstorage material 31 or the heat transfer medium 47 as in the cases shownin FIGS. 11 and 12, and other heat storage methods mainly utilize atransition heat of the heat storage material 51 as in the case shown inFIG. 13. Whichever heat of a sensible heat or a transition heat may beutilized, in order to store the heat of, for example, a temperature ofT_(x), it is necessary for the surface temperature T_(x)′ of the heatstorage material to be maintained at a temperature of at least T_(x).However, the greater is the temperature difference between the surfacetemperature T_(x)′ of the heat storage material and the ambienttemperature T₀ in the outside, and also the longer is the storage time,the larger the heat loss represented in the equation (2) becomes, whichresults in a marked reduction in heat storage efficiency.

In order to reduce the heat loss from the heat storage tank in theequation (2), therefore, it is necessary to reduce the overall heattransfer coefficient k and/or the surface area A and shorten the time t,or to reduce the difference between the surface temperature of the heatstorage material T_(x)′ and the environmental temperature T₀.

As a method for reducing the overall heat transfer coefficient k, anattempt to install a heat insulator around the periphery of the heatstorage tank has been made. As a method for reducing the surface area,there has been made an attempt to make the heat storage tank have ashape having a small surface area per a unit volume, such as a cube or asphere.

As a method for shorten the time t, there has been made an attempt tooptimize the system control over heat application system after there-extraction of heat. However, since the time term and temperature termin the equation (2) are associated with the original purpose of the heatstorage, they can not be widely changed by nature.

On the other hand, in the examples shown in FIGS. 11 and 12, since thetemperature of the heat storage material gradually approaches that ofthe external environment from the portions closer to the wall surfacesof the heat storage tanks 32 and 42, a temperature difference isgenerated within the heat storage material. In general, since a matterexpands or contracts as it changes in temperature, once a temperaturedifference is generated in the heat storage material, a densitydifference occurs therein, and a gravity difference is caused. As aconsequence, a movement called a natural convection is induced withinthe heat storage material.

Therefore, a natural convection takes place from the vicinity of thewall surfaces of the heat storage tank 32 and 42 that have the highesttemperature difference between the same and the external environment,that is, from the outer portions in the heat storage material, and itspreads throughout the heat storage material. Consequently, heat storagematerial having a temperature closer to the temperature of externalenvironment rather than to the heat stored flows from the surface sideinto the inner portion of the heat storage material, and simultaneously,the heat stored in the surface of the heat storage material transfers,whereby heat transfer always takes place between the heat storagematerial and the external environment through the wall surface of theheat storage tank, and hence causes a large heat loss within the heatstorage material.

In the above-described heat storage device shown FIG. 13, in theconservation process of heat, the natural convection of the heat storagematerial 51 within the heat storage tank 52 is suppressed by smallvessels 56, but a natural convection is generated within the heattransfer medium 57 stagnating in the heat storage tank 52. Accordingly,with regard to the heat transfer medium 57, the same as the case of theheat storage materials 31 and 47 of the heat storage devices shown inFIGS. 11 and 12 holds true.

In this way, since the temperature of the heat storage material changesto the temperature close to that of the external environment from theheat storage material in the portion closer to the wall surface of heatstorage tanks, a temperature difference is generated between the outerand inner portions within the heat storage material (in the case ofspherical heat storage tank, in the radial direction) in the process ofheat transfer. Consequently, since a natural convection is inducedwithin the heat storage material, heat transfer always takes placebetween the heat storage material and the external environment throughthe wall surface of heat storage tank, incurring a heat loss of the heatstorage material.

In the above-described conventional heat storage devices shown in FIGS.11 through 13, since the heat storage materials tend to easily flow inany case, a natural convection throughout the heat storage material isprone to occur not only in the process of conservation of heat, but alsoparticularly in the processes of the injection and extraction of heatdue to a temperature difference as a result of heat exchange. Thisincurs an increase in the overall heat transfer coefficient k in theequation (2), and an increase in the heat loss from the heat storagematerial to the outside, which constitutes one of the factors reducing aheat recovery rate.

SUMMARY OF THE INVENTION

The present invention has been achieved to overcome the heat loss from aheat storage material as described above and aims to provide a heatstorage device capable of reducing the influence of external environmenton the inside portion of a heat storage tank and suppressing the heatloss toward the outside, by reducing the temperature difference betweenthe surface of the heat storage material and the external environment,and at the same time, by suppressing the heat transfer between thesurface of the heat storage material and the inside portion of the heatstorage material which executes injection and extraction between thesame and the external environment.

A first heat storage device for achieve the above-mentioned objectcomprises: a heat storage tank charged with a heat storage material forstoring the heat provided from the outside; and heat exchange meanswhich executes the injection and extraction of heat between the insideof a heat storage tank and the outside by the heat exchange between theheat storage material and the heat transfer medium, wherein the heatexchange means is disposed so as to execute a heat exchange between thecentral portion of the heat storage tank and the outside, or so that thecentral portion and the outer portion in the heat storage tank beingcaused to perform a heat exchange with the outside sequentially orindividually, and wherein suppressing means are disposed in the outerportion in the heat storage tank, for suppressing the natural convectionof the heat storage material.

A second heat storage device for achieve the above-mentioned objectcomprises: a heat storage tank charged with a heat transfer medium whichalso serves as a heat storage material for storing heat supplied fromthe outside; and heat transport means which executes an injection andextraction of heat between the inside of the heat storage tank and theoutside by the inflow and outflow of the heat transfer medium, whereinthe heat transport means is disposed so as to execute a heat transportbetween the central portion of the heat storage tank and the outside,and wherein suppressing means are disposed in the outer portion in theheat storage tank, for suppressing the natural convection of the heatstorage material.

In the above-described heat storage devices, suppressing means may beconstituted by: dispersing a liquid-absorbent material into the heatstorage material or heat transfer medium in the outer portion in theheat storage tank; providing the heat storage material or heat transfermedium in the outer portion in the heat storage tank with a property ofincreasing viscosity by the application of voltage, and providing theheat storage device with means for applying power between a pair ofelectrodes which are disposed on opposite sides of the outer portion inthe heat storage tank so as to sandwich the outer portion; or providingthe heat storage material or heat transfer medium in the outer portionin the heat storage tank with a property of increasing viscosity by theapplication of magnetic force, and providing the heat storage devicewith a magnet for exerting a magnetic force on the outer portion in theheat storage tank; and further providing the outer portion in the heatstorage tank with a barrier for hindering the natural convection of theheat storage material or heat transfer medium.

Also, in the above-described heat storage devices, there can be providedmeans for promoting the heat transfer between the central portion andthe outer portion in the heat storage tank.

In the heat storage device having the above-described constitution, theheat of a temperature more than necessary at the heat extraction isinjected to the heat storage tank by heat exchange means which execute aheat exchange with the central portion in the heat storage tank or byheat transport means which transport heat to the central portion, andthen the heat is transferred to, stored in, and conserved in the heatstorage material or heat transfer medium which is accommodated mainly inthe outer portion in the heat storage tank. As necessary, the heatstored is extracted from the central portion of heat storage tank.

When injecting or extracting heat, since a heat exchange is executedmainly in the central portion in heat storage tank where convectioneasily occurs, a large heat transfer due to natural convection heattransfer joins in the heat transfer due to a heat conduction, andthereby an efficient heat exchange with the outside is achieved.

When conserving heat, since a heat transfer is usually generated betweenthe outer portion in the heat storage tank and the external environment,a temperature difference due to heat transfer is generated in the heatstorage tank, and consequently, a natural convection tries to arise fromthe heat storage material or heat transfer medium located closer to theheat storage tank. However, in the above-described heat storage devices,the suppressing means for suppressing a natural convection, disposed inthe outer portion in the heat storage tank, make it difficult for theouter portion to perform a large heat transfer due to naturalconvection.

As described above, in the present invention, the natural convection issuppressed in the outer portion in the heat storage tank, and thenatural convection generated on the surface of the heat storage materialin the outer portion in the heat storage tank is hindered from extendingthroughout the heat storage material in the outer portion in the heatstorage tank, and consequently, the heat transfer and overall heattransfer coefficient (k) due to the convection between the outer portionin the heat storage tank and the external environment is reduced, and atthe same time, the heat within the heat storage material in the outerportion in the heat storage tank is hindered from transferring to thesurface of the heat storage material. This creates a great temperaturegradient within the heat storage material in the outer portion, andgradually reduces the temperature gradient between the surface of heatstorage material and the external environment.

It is therefore possible to reduce the heat loss from the inside of theheat storage tank to the external environment while maintaining thetemperature of the inside of the heat storage tank substantiallyconstant.

In the present invention, the suppressing means against naturalconvection can be constituted by dispersing a liquid-absorbent materialinto the heat storage material or heat transfer medium in the outerportion in the heat storage tank. In this case, the above-mentionedliquid-absorbent material stagnates in the form of three-dimensionalmeshes in the outer portion in the heat storage tank, by adsorbing apart of the heat storage material or heat transfer medium and expanding.That is, since the heat storage material or the like comes to fill gapswithin the liquid-absorbent material so as to be difficult to move, thenatural convection of the heat storage material in the outer portion issuppressed, which enables the reduction in the heat loss from the insideof the heat storage tank to the outside.

As the above-mentioned suppressing means, when providing the heatstorage material or heat transfer medium in the outer portion in theheat storage tank with a property of increasing viscosity by theapplication of voltage, and means for applying power between the pair ofelectrodes which are disposed on opposite sides of the outer portion inthe heat storage tank so as to sandwich the outer portion, an electricfield is generated in the outer portion by the application of voltage,and thereby the viscosity of the heat storage material is increased.This hinders a free movement of the heat storage material, and allowsthe suppression of the natural convection of the outer portion. Also, assuppressing means, when providing the heat storage material or heattransfer medium in the outer portion in the heat storage tank with aproperty of increasing viscosity by the application of a magnetic force,and providing a magnet for exerting a magnetic power on the outerportion in the heat storage tank, the viscosity of the heat storagematerial or the heat transfer medium is increased by the action of themagnetic field of the magnet. This hinders a free movement of the heatstorage material, and allows the suppression of natural convection ofthe outer portion.

Further, the suppressing means can be constituted as a barrier disposedin the outer portion in the heat storage tank, for hindering the naturalconvection of the heat storage material or the heat transfer medium. Inthis case, since the movement of the heat storage material or heattransfer medium can physically be hindered by the barrier, appropriatelydisposing the barrier permits the moving speed of the heat storagematerial to be significantly lower than the case without hindrance,which results in the suppression of natural convection.

In addition, in the present invention, means for promoting heat transferis provided between the central portion and the outer portion in theheat storage tank. When, for example, a rapid injection to and anextraction from the heat storage tank is required, it is possible tospeedily execute a heat transfer between the central portion and theouter portion where the heat transfer from the central portion isdifficult due to the suppression means against natural convection.

The heat exchange means in the present invention can be disposed so thatthe central portion and the outer portion in the heat storage tank arecaused to sequentially perform a heat exchange with the outside. By thisconstitution, when injecting heat, by utilizing the residual heatremaining after the heating or cooling of the central portion in theheat storage tank, it is possible to additionally heat or cool the outerportion. On the contrary, when extracting heat, after the preheating orprecooling of the heat exchanger in the outer portion in the heatstorage tank, it is possible to additionally heat or cool the heatexchanger in the outer portion. This allows the exchange of heat betweenthe heat storage device and the external environment to be executedrapidly and efficiently.

Moreover, the heat exchange means can be disposed so that the centralportion and the outer portion in the heat storage tank are caused toindividually perform a heat exchange with the outside. By thisconstitution, when injecting heat, a heat exchange can be executed inthe central portion or the outer portion whichever is more suitable, inaccordance with the temperature of the heat injected from the outside tothe heat storage tank. On the contrary, when extracting heat, a heatexchange may be executed in the central portion or the outer portionwhichever is more suitable, in accordance with the temperature of theheat required by the outside. Consequently, the exchange of heat betweenthe heat storage device and the external environment can be executedrapidly and efficiently.

In accordance with the above-described heat storage devices associatedwith present invention, since the ease of convection of the heat storagematerial or heat transfer medium within the heat storage tank variesdepending on position, the injection operation from the outside to theheat storage material and the a extraction operations from the heatstorage material to the outside are executed toward the central portionwhere a natural convection is easy to occur, while the storage of heatis executed in the central portion where a natural convection isdifficult to occur. Therefore, in the processes of the injection andextraction of heat as well as in the process of conservation, thenatural convection occurring within the inside of the heat storagematerial due to the temperature difference from the outside takes placein the central portion in the heat storage tank and hardly takes placein the outer portion, with the result that the overall heat transfercoefficient k becomes less than conventional cases. This enables theprovision of a heat storage device having a low heat loss toward theoutside.

Also, providing the central portion and the outer portion with heattransfer elements permits the heat transfer between the central portionand the outer portion where heat transfer does not much occur basicallyto be temporarily promoted, whereby it is possible to provide a heatstorage device having a low heat loss as described above while executingan efficient operating of the heat storage tank in accordance with thesupply and demand of heat.

Further, disposing heat exchangers so as to make a circuit through thecentral portion and the outer portion in the heat storage tank, permitsthe heat exchanger to execute a direct heat exchange with the outerportion where heat transfer does not much occur basically, and toexecute an injection or an extraction of heat between the centralportion and the outer portion sequentially, whereby it is possible toprovide a heat storage device having a low heat loss as described abovewhile executing the exchange of heat between the heat storage device andthe outside rapidly and on a large scale. Also, individually disposingheat exchangers in the central portion and the outer portion enables aselective use of either the central portion or the outer portion eachhaving a different temperature, or both of them, whereby it becomespossible to provide a heat storage device having a low heat loss whileefficiently executing an heat change between the heat storage device andthe outside in accordance with the supply and demand of heat.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 through 10 are sectional views showing varied embodiments ofheat storage devices of the present invention.

FIGS. 11 through 13 are sectional views showing varied examples ofconventional heat storage devices.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiment 1

FIG. 1 is an embodiment of sectional view of the heat storage device inaccordance with the present invention. This heat storage deviceaccommodates a heat storage material 1 for storing heat in a heatstorage tank 2 and has a heat exchanger 3 in a heat storage tank 2, asmeans for executing a heat exchange with the outside. As a heat storagematerial 1, various materials may be used in accordance with a requiredheat storage temperature. The heat storage material 1 may be a materialwhich makes a phase change in the processes of injection and extractionof heat. The inside space of the heat storage tank is divided by a net 4into a central portion 2 a and an outer portion 2 b surrounding thecentral portion 2. That is, within the heat storage tank 2, the net 4 isstretched throughout the boundary between the central portion 2 a andthe outer portion 2 b, and the central portion 2 a is surrounded by theouter portion 2 b via the net 4. A liquid-absorbent material 5 isdispersed only in the heat storage material 1 existing in the outerportion.

It is preferable that the above-mentioned net 4 has a stability towardthe heat storage material 1 and the liquid-absorbent material 5, andprevents the liquid-absorbent material 5 from mixing into the centralportion 2 a.

The heat exchanger 3 is provided as means for executing a heat exchangebetween the outside of the heat storage device and the heat storagematerial 1 existing in the central portion 2 a, and its material andstructure and the like are not limited as long as it is stable towardthe heat storage material 1. Preferably, the liquid-absorbent material 5is stable chemically and thermally toward the heat storage tank 2 andthe net 4, and increases the viscosity of the heat storage material 1existing in the outer portion 2 b, whereby it can constitute suppressingmeans against natural convection. For such materials, various materials,for example, polymer material particles with a high water-absorbencysuch as starch-acrylonitrile graft polymeric hydrolysate, and fiber suchas absorbent cotton are applicable.

Next, a description is given of the operation of the heat storage deviceshown in FIG. 1, for the case where a higher temperature than that ofenvironment outside the heat storage device is stored.

In the injection process of heat, out of the heat storage material 1charged into the heat storage tank 2, the one existing in the centralportion 2 a is heated to raise temperature, using the heat exchanger 3.Once the temperature of the heat storage material 1 of the centralportion 2 a is raised, the heat transfers from the central portion 2 ato the outer portion 2 b, and the heat storage material 1 existing inthe outer portion 2 b is gradually heated.

In this case, also in the injection and extraction processes of heat,temperature transfers from the outer portion 2 b to the externalenvironment surrounding the heat storage tank 2 through the wall of theheat storage tank 2. However, for the sake of simplicity, the heat losstoward the outside is described only in respect of the conservationprocess.

In the conservation process of heat, since heat transfers from thecentral portion 2 a to the external environment surrounding the heatstorage tank 2, the temperature of the heat storage material 1 graduallydecreases. As described above, the temperature of the heat storagematerial 1 changes from the outer portion closer to the heat storagetank 2, and hence, in the heat transfer process, a temperaturedifference is generated in the direction from the central portion to theouter portion within the heat storage material 1. In this case, if theheat storage material 1 is of a free fluidity, a natural convection willbe induced within the heat storage material 1.

It is well known that the strength of this natural convection iscontrolled by the Grashof number Gr defined by the following equation.$\begin{matrix}{{G\quad r} = \frac{L^{3}g\quad \beta \quad \Delta \quad T}{v^{2}}} & (3)\end{matrix}$

L: representative length(e.g., the height of vertical wall surface ofthe heat storage tank 2).

g: acceleration of gravity.

β: coefficient of volume expansion of the heat storage material 1.

ΔT: temperature difference generated within the heat storage material 1.

ν: coefficient of kinematic viscosity of the heat storage material 1.

If there is no mass transfer within the heat storage material 1, theheat transfer between the heat storage material 1 and heat storage tank2 will depend only on the direct heat transfer between componentmolecules/atoms which are adjacent to each other in the heat storagematerial 1 and the heat storage tank 2, that is, only on heatconduction. However, if there is an above-described natural convectionwithin the heat storage material 1, there will exist, in addition toheat conduction, a fast and large heat transfer (heat transfer) due tonatural convection, or mass transfer, and thus a free and active heattransfer will occur. It is well known that the strength of the heattransfer between a fluid and a solid wall surface is controlled by theNusselt number Nu defined by the following equation. $\begin{matrix}{{N\quad u} = {\frac{h\quad x}{\lambda} = {C\left( {P\quad r\quad G\quad r} \right)}^{m}}} & (4)\end{matrix}$

h: coefficient of heat transfer between the heat storage material 1 andthe heat storage tank 2.

x: distance from the bottom of the vertical wall surface of the heatstorage tank 2.

λ: coefficient of heat conduction of the heat storage material 1.

Pr: Prandtl number of the heat storage material 1 (Pr=ν/α, where αdenotes the thermal diffusivity of the heat storage material 1).

C, m: constants determined by the state of a wall surface.

The right side of the equation (4) shows that Nusselt number Nu is afunction of Prandtl number Pr and Grashof number. From the equations (3)and (4), it is seen that the coefficient of heat transfer between theheat storage material 1 and the heat storage tank 2 is expressed by:$\begin{matrix}{h = {C\frac{\lambda}{x}\left( \frac{L^{3}g\quad \beta \quad \Delta \quad T}{\alpha \quad v} \right)^{m}}} & (5)\end{matrix}$

The equation (5) shows that, the higher the coefficient of heatconduction, the coefficient of volume expansion of the heat storagematerial 1, the vertical length of the heat storage tank 2, or thetemperature difference between the heat storage material 1 and the heatstorage tank 2, the higher the magnitude of the coefficient ofheat-transfer h, that is, the heat transfer between the heat storagematerial 1 and the heat storage tank 2 becomes, and that, on thecontrary, the higher the thermal diffusivity, or the coefficient ofkinematic viscosity of the heat storage material 1, the lower thetransfer between the heat storage material 1 and the heat storage tank 2becomes.

The quantity of heat transfer from the heat storage material 1 to theoutside follows the equation (2), and the higher the coefficient of heattransfer h between the heat storage material 1 and the heat storage tank2, the higher the heat loss from the heat storage material 1 to theexternal environment becomes, because the overall heat transfercoefficient k becomes higher responding the sensitively to such anincrease in the coefficient of heat transfer h.

In the heat storage device of this embodiment, since the heat storagematerial 1 existing in the outer portion 2 b undergoes an action that isequal to the action of increasing viscosity ν from the liquid-absorbentmaterial 5 dispersed in the outer portion 2 b, a natural convectionbecomes difficult to arise as suggested by equation (3), and as aconsequence, the heat transfer between the heat storage material 1existing in the outer portion 2 b and the heat storage tank 2 decreasesas suggested by the equation (5), leading to a lower heat loss towardthe outside.

In the traction process of heat, it is possible to extract the heatcorresponding to the temperature of the heat storage material 1, fromthe heat storage tank 2, using the heat exchanger 3. As the temperatureof the heat storage material 1 existing in the central portion 2 adecreases as a result of the heat extraction, the heat storage material1 existing in the outer portion 2 b is gradually cooled.

Embodiment 2

FIG. 2 is a sectional view of another embodiment of the heat storagedevice in accordance with the present invention. This heat storagedevice accommodates a heat transfer medium 8 which also serves as a heatstorage material, in a heat storage tank 2. However, unlike the case ofFIG. 1, this heat storage device makes communicate the central portion 2a surrounded by a net 4 in the heat storage tank 2, with the outside ofthe heat storage device by two communicating tubes 6 a and 6 b whichexecute the inflow and outflow of the transfer medium 8. The otherconstructions are similar to the embodiment shown in FIG. 1.

Next, a description is presented of the operation of the heat storagedevice in the case where a higher temperature than that of the externalenvironment is stored in the heat storage device shown in FIG. 2.

In the injection process of heat, the heat transfer medium 8 sucked outfrom the communicating tube 6 b is heated by a heat source at theoutside, and is circulated back to the heat storage tank 2 using thecommunicating tube 6 b. As a consequence, the temperature of the heattransfer medium 8 existing in the central portion 2 a rises, with theresult that heat is injected to the heat storage tank 2. As thetemperature of the heat transfer medium 8 in the central portion 2 arises, the heat transfer medium 8 in the outer portion 2 b is graduallyheated.

In the conservation process of heat, since heat transfers to theexternal environment surrounding the heat storage tank 2, thetemperature of the heat transfer medium 8 gradually decreases. However,as in the case of the operation in FIG. 1, a natural convection becomesdifficult to occur due to the liquid-absorbent material 5 dispersed inthe outer portion 2 b, and consequently, the heat transfer between theheat transfer medium 8 existing in the outer portion 2 b and the heatstorage tank 2 decreases, leading to a lower heat loss toward theoutside.

In the traction process of heat, the heat transfer medium 8 accommodatedin the heat storage tank 2 is sucked out from the communicating tube 6a, and is utilized at the outside as a heat source having thetemperature of the heat transfer medium 8. The heat transfer medium 8cooled as a result of the thermal utilization is circulated back to theheat storage tank 2 through the communicating tube 6 b. As thetemperature of the heat transfer medium 8 existing in the centralportion 2 a decreases as a result of the heat extraction, thetemperature of the heat transfer medium 8 existing in the outer portion2 b is gradually cooled.

Embodiment 3

FIG. 3 is a sectional view of another embodiment of the heat storagedevice in accordance with the present invention. In this heat storagedevice, in a heat storage tank 2 having a similar construction as theembodiment shown in FIG. 2, heat storage materials 1 a and 1 b to beaccommodated in the central portion 2 a and the outer portion 2 b areaccommodated in small vessels 7 a and 7 b, respectively. As heat storagematerials 1 a and 1 b, various materials may be used in accordance withrequired temperatures. The heat storage material may be a material whichmakes a phase change in the processes of injection and extraction.Further, the heat storage materials 1 a and 1 b which exist respectivelyin the central portion 2 a and the outer portion 2 b in a heat storagetank 2 may be an identical material, or may be different materials.

In the drawing, though small vessels 7 a and 7 b in this embodiment arerepresented by a spherical shape, the material and shape of the smallvessel are not limited, as long as the small vessels are stable towardthe heat storage materials 1 a and 1 b and the heat transfer medium 8,and the dimensions of the small vessels may be different between thosein the central portion 2 a and those in the outer portion 2 b, or may bethe same.

Next, a description is given of the operation of the heat storage devicein the case where a higher temperature than that of environment outsidethe heat storage device is stored in the heat storage device shown inFIG. 3.

In the injection process of heat, as in the case of the embodiment shownin FIG. 2, the heat transfer medium 8 heated at the outside iscirculated back to the heat storage tank, and consequently, heattransfers from the heat transfer medium 8 to the heat storage materials1 a, the temperature thereof rising, thereby heat being injected to theheat storage tank 2. As the temperature of the heat storage material 1 aexisting in the central portion 2 a rises, the heat storage material 8existing in the outer portion 2 b is gradually heated.

In the conservation process of heat, since heat transfers to theexternal environment surrounding the heat storage tank 2, thetemperature of the heat transfer medium 8 gradually decreases. However,as in the case of the operation in FIG. 1, it becomes difficult for anatural convection to occur due to the liquid-absorbent material 5dispersed in the outer portion 2 b, and consequently, the heat transferbetween the heat transfer medium 8 existing in the outer portion 2 b andthe heat storage tank 2 decreases, leading to a lower heat loss towardthe outside.

The traction process of heat is substantially the same as that of theembodiment shown in FIG. 2.

Embodiment 4

FIG. 4 is a sectional view of another embodiment of the heat storagedevice in accordance with the present invention. Although this heatstorage device has substantially the same construction as that of theembodiment shown in FIG. 1, it has a thermal insulating layer 9 outsidea heat storage tank 2. Therefore, the description of the common partswith the embodiment shown in FIG. 1 is omitted, and the commonreferential numerals are marked with the identical numerals in FIG. 4.

Next, a description is given of the operation of the heat storage devicein the case where a higher temperature than that of environment outsidethe heat storage device is stored in the heat storage device shown inFIG. 4.

The operation in the injection process of heat is basically the same asthat of the embodiment 1.

Although the operation in the conservation process of heat is alsobasically the same as that of the embodiment 1, the coefficient of heatconduction of the thermal insulating layer 9 is lower than that of theheat storage tank 2, so that it is possible to reduce the overall heattransfer coefficient k shown in the equation (2) and to provide a heatstorage device with a further lower heat loss.

The operation in the extraction process of heat is basically the same asthat of the embodiment 1.

In this embodiment, a case is shown where the heat storage tank and thelike is same as those in the embodiment 1, but these operations areapplicable also for such constructions as shown in the embodiment 2 or3.

Embodiment 5

FIG. 5 is a sectional view of another embodiment of the heat storagedevice in accordance with the present invention. Although this heatstorage device has a similar heat storage tank 2 and a heat exchanger 3to the embodiment shown in FIG. 1, it has an electrode 4 a so as tospatially separate the central portion 2 a and the outer portion 2 b, inplace of the net 4 shown in FIG. 1. Also, an electrode 4 b is stretchedaround in the vicinity of the inner surface of the heat storage tank 2.The electrodes 4 a and 4 b are disposed on opposite sides so as tosandwich the outer portion 2 b, each connected to a power supply 11.Inside the heat storage tank 2, an electrorheological fluid 10 ischarged, which has a property of increasing viscosity by application ofvoltage and which serves as a heat storage material.

The material and structure of the electrodes 4 a and 4 b are notlimited, as long as they are capable of generating a uniform electricfield in the outer portion 2 b by the application of voltage to thepower supply 11. They may, for example, be reticular. Preferably, theelectrorheological fluid 10 is stable toward the heat storage tank 2,the heat exchanger 3, the electrodes 4 a and 4 b. For anelectrorheological fluid, various materials such as, for example, aliquid which is suspending carbon particles are applicable in accordancewith required heat storage temperatures.

Next, an explanation is presented of the operation of the heat storagedevice in the case where a higher temperature than that of the externalenvironment is stored in the heat storage device shown in FIG. 5.

In the injection process of heat, out of the electrorheological fluid(heat storage material) 10 accommodated in the heat storage tank 2, theone existing in the central portion 2 a is heated to raise temperature,using the heat exchanger 3, and thereby the electrorheological fluid 10existing in the outer portion 2 b is gradually heated.

If the quantity of heat supplied from the outside to the heat storagedevice is lower than or equal to that of the quantity of heat diffusingfrom the central portion 2 a to the outer portion 2 b, then a voltage isapplied between 4 a and 4 b by the power supply 11. As a consequence, anelectric field is generated between 4 a and 4 b, and the viscosity ofthe electrorheological fluid 10 existing in the outer portion 2 bsandwiched by the electrodes 4 a and 4 b is increased, resulting in alower heat loss toward the outside.

If the quantity of heat supplied from the outside to the heat storagedevice is larger than the quantity of heat diffusing from the centralportion 2 a to the outer portion 2 b, then it will be possible to reducethe viscosity of the outer portion 2 b by not applying a voltage betweenthe electrodes 4 a and 4 b, and to allow the heat transfer from thecentral portion 2 a to the outer portion 2 b to freely take place,thereby causing the heat injected from the outside to be absorbedefficiently into the heat storage tank 2.

In the conservation process of heat, a voltage is applied to theelectrodes 4 a and 4 b by the power supply 11. The application of avoltage reduces the heat loss from the heat storage device to theoutside for the foregoing reason.

In the extraction process of heat, the heat corresponding to thetemperature of the electrorheological fluid 10 can be extracted from theheat storage tank 2 through the heat exchanger 3. As the temperature ofthe electrorheological fluid 10 in the outer portion 2 b decreases as aresult of the extraction of heat, the electrorheological fluid 10 in thecentral portion 2 a gradually cooled.

If the quantity of heat required of the heat storage device by theoutside is met by the heat conserved in the central portion 2 a, avoltage will be applied to the electrodes 4 a and 4 b by the powersupply 11. The application of a voltage reduces the heat loss from theheat storage device to the outside for the foregoing reason. If thequantity of heat required of the heat storage device by the outside isnot met by only the heat conserved in the central portion 2 a, it willbe possible to reduce the viscosity of the outer portion 2 b by notapplying a voltage between the electrodes 4 a and 4 b, and to allow theheat transfer from the outer portion 2 b to the central portion 2 a tofreely take place, thereby causing the heat needed at the outside to beextracted efficiently from the heat storage tank 2.

Further, as in the case of embodiment 4, a heat storage device having alower heat loss can be achieved by covering the heat storage tank withthe thermal insulating layer 9.

Embodiment 6

FIG. 6 is a sectional view of another embodiment of the heat storagedevice in accordance with the present invention. This heat storagedevice has a similar heat storage tank 2 and heat exchanger 3 to theembodiment shown in FIG. 1, but in the heat storage tank 2 a magneticfluid 12 is charged, which has a property of increasing viscosity by theapplication of a magnetic field.

On the boundary between the central portion 2 a and the outer portion 2b in the heat storage tank 2, and outside the heat storage tank 2, amultitude of electromagnets 13 a and 13 b which generate a magneticforce by a application of an external power supply, are disposed by anarbitrary number. In the drawing, the wiring between the electromagnets13 a and 13 b and the power supply 11 are omitted.

The electromagnet 13 a is disposed so that the magnetic poles face awayfrom the central portion 2 a, and on the contrary, the electromagnet 13b is disposed so that the magnetic poles face away from the outside ofthe heat storage device. The material and structure of electromagnets 13a and 13 b are not limited as long as they can generate a magnetic fieldonly in the outer portion. For example, permanent magnets may be used inplace of the electromagnets 13 a and 13 b. Also in that case, the basicoperation is the same. For an electrorheological fluid 12, variousmaterials, such as a liquid which is suspending carbon particles, areapplicable in accordance with required heat storage temperatures.

The processes of injection, conservation and extraction of heat in theheat storage device shown in FIG. 6 are the same as those in theembodiment shown in FIG. 5, except that the viscosity of the magneticfluid 12 is increased by the generation of the magnetic field throughthe energizing of the electromagnets 13 a and 13 b in place of thegeneration of electric field.

The operation in the case where the electromagnets 13 a and 13 b arereplaced with permanent magnets is the same as that of the case wherethe electromagnets 13 a and 13 b are energized. Further, as in the caseof the embodiment 4, a heat storage device with a lower heat loss canalso be achieved by covering the heat storage tank with the thermalinsulating layer 9.

Embodiment 7

FIG. 7 is a sectional view of another embodiment of the heat storagedevice in accordance with the present invention. This heat storagedevice accommodates a heat storage material 1 for storing heat, in aheat storage tank 2 thereof, and has a heat exchanger 3 for executing aheat exchange with the outside, in the heat storage tank 2, as in thecase of the embodiment shown in FIG. 1. However, it has a multitude ofbarriers 14 disposed in the outer portion 2 b partitioned by a net 4 inthe a heat storage tank 2. For the barriers 14, for example, reticularones may be applicable.

In this embodiment, a liquid-absorbent material 5 used in the embodimentshown in FIG. 1 is not used, but it may be employed. Because otherconstructions are the same as the embodiment shown in FIG. 1,description of them is omitted.

Next, an explanation is presented of the operation of the heat storagedevice in the case where a higher temperature than that of environmentoutside the heat storage device is stored in the heat storage deviceshown in FIG. 7.

The operation in the injection process of heat is basically the same asthat of the embodiment 1. In the conservation process of heat, sinceheat transfers from the central portion 2 a to the external environment,the temperature of the heat storage material 1 gradually decreases.However, even if the heat storage material 1 of the outer portion 2 btries to transfer to the central portion 2 a, it will repeatedly collideagainst the minutely intricated barriers 14, so that the moving speed ofthe heat storage material 1 becomes lower than the case without thebarriers 14, and thereby the heat storage material 1 undergoes an actionthat is apparently equal to the action of increasing viscosity ν of theheat storage material 1 existing in the outer portion 2 b, which leadsto a lower heat loss toward the outside.

The operation in the extraction process of heat is also basically thesame as that of the embodiment 1. Further, as in the case of theembodiment 4, a heat storage device having a lower heat loss can beachieved by covering the heat storage tank with the thermal insulatinglayer 9.

Embodiment 8

FIG. 8 is a sectional view of another embodiment of the heat storagedevice in accordance with the present invention. This heat storagedevice has basically the same construction as the embodiment shown inFIG. 1, and in the outer portion 2 b partitioned by a net 4 in a heatstorage tank 2, a liquid-absorbent material 5 is dispersed in a heatstorage material 1. Further, a heat transfer element 15 capable ofarbitrarily promoting heat transfer is disposed between the centralportion 2 a and the outer portion 2 b in the heat storage tank. The heattransfer element 15 may be, for example, one capable of transporting anyquantity of heat in any direction by the application of voltage, or maybe one which markedly varies in the heat transfer resistance withtemperature, such as a heat pipe.

Next, an explanation is presented of the operation of the heat storagedevice in the case where a higher temperature than that of the externalenvironment is stored in the heat storage device shown in FIG. 8.

The operation in the injection process of heat is basically the same asthe case of the embodiment 1. However, if the quantity of heat suppliedby the outside is more than heats the heat storage material 1 of thecentral portion 2 a, it is possible, in addition to the procedure in theembodiment 1, to promote the heat transfer from the central portion 2 a,to the outer portion 2 b by the heat transfer element 15, and therebystore the heat supplied by the outside in the heat storage device ascompletely and rapidly as possible. The operation in the conservationprocess of heat is also similar to that of the embodiment 1, and theheat loss from the heat storage device is less than conventional cases.

The operation in the extraction process of heat is also basically thesame as that of the embodiment 1. However, if the quantity of heatsupplied by the outside is not met by only the use of the heat stored inthe heat storage material 1 existing in the central portion 2 a, it ispossible, in addition to the procedure in the embodiment 1, to promotethe heat transfer from the outer portion 2 b to the central portion 2 aby the heat transfer element 15, and thereby to extract the heatrequired by the outside from the heat storage device as completely andrapidly as possible.

In this embodiment, a case where the heat storage tank and the like aresimilar to those in the embodiment 1 is shown, but similar operations tothe foregoing can be achieved also in the case of such constructions asshown in the embodiments 2 through 7.

Embodiment 9

FIG. 9 is a sectional view of another embodiment of the heat storagedevice in accordance with the present invention. This heat storagedevice has basically the same construction as the embodiment shown inFIG. 1, and provides a heat storage tank 2 with an heat exchanger 16disposed so as to sequentially pass the central portion 2 a and theouter portion 2 b, for executing a heat exchange between the outside.Also, a liquid-absorbent material 5 is dispersed in the heat storagematerial 1 of the outer portion 2 b partitioned by a net 4 in the heatstorage tank 2. With regard to the heat exchanger 16, the constitutionssuch as material, the cross-sectional profile, and the presence orabsence of a fin is not limited, as long as the heat exchanger 16 has astability toward the heat storage material 1.

Next, an explanation is presented of the operation of the heat storagedevice in the case where a higher temperature than that of the externalenvironment is stored in the heat storage device shown in FIG. 9.

The operation in each process is basically the same as that of theembodiment 1. In the injection process of heat, firstly the centralportion 2 a of the heat storage material 1 charged into heat storagetank 2 is heated to increase temperature using the heat exchanger 16.Next, the heat storage material 1 of the outer portion 2 b is heated byusing the residual heat. In the embodiment 1, the heat storage material1 of the outer portion 2 b is gradually heated through the heat transferdue to the temperature increase of the heat storage material 1 of thecentral portion 2 a, but the outer portion 2 b has a high apparentviscosity as described in the embodiment 1, so that a heat transfer isdifficult to occur. By utilizing the heat exchanger 16, a more rapidinjection of heat than the case of the embodiment 1 can be achieved.

The conservation process of heat is similar to that of the embodiment 1.

In the extraction process of heat, by using the heat existing in theouter portion 2 b out of the heat storage material 1 charged into theheat storage tank 2, the heat transfer medium to perform a heat exchangewith the outside are heated by way of preheating. Next, the heattransfer medium is heated by the heat of the heat storage material 1 ofthe central portion 2 a. In this embodiment, as in the case of theembodiment 1, the outer portion 2 b is high in apparent viscosity so asto be difficult to perform heat transfer. However, use of the heatexchanger enables a more rapid injection of heat than the case of theembodiment 1.

In this embodiment, a case where the heat storage tank and the like aresimilar to those in the embodiment 1 is shown, but similar operations tothe forgoing can be achieved also when applied to such structures asshown in the embodiments 2 through 8.

Embodiment 10

FIG. 10 is a sectional view of another embodiment of the heat storagedevice in accordance with the present invention. This heat storagedevice has heat exchangers 17 a and 17 b capable of heating or coolingthe central portion 2 a and the outer portion 2 b in a heat storage tank2 individually, in place of the heat exchanger 16 in the embodimentshown in FIG. 9. Since the material of the heat exchanger 17 a and 17 bare the same as those of the embodiment shown in FIG. 9 and the otherconstructions are also the same as those of the embodiment shown in FIG.9, description of constructions is omitted.

Next, an explanation is presented of the operation of the heat storagedevice in the case where a higher temperature than that of the externalenvironment is stored in the heat storage device shown in FIG. 10.

The operation in each process is basically the same as that of theembodiment 1. In the injection process of heat, procedure varies inaccordance with the following three cases.

A first procedure is adopted when the temperature of the heat source ishigher than that of the central portion 2 a of the heat storage material1, and at the same time, when the quantity of the heat of the heatsource is more than heats the central portion 2 a. At this time, theheat transfer medium which exchanges heat with the outside is first sentfrom the heat source to the heat exchanger 17 a, and heats the heatstorage material 1 of the central portion 2 a, being then returned tothe outside. However, since the temperature of this heat transfer mediumreturned is higher than that of the heat storage material 1 of the outerportion 2 b, the heat transfer medium is next sent to the heat exchanger17 b, and heats the heat storage material 1 of the outer portion 2 b,being then returned to the outside. This makes it possible to rapidlyheat the heat storage material 1 of the outer portion 2 b and to improvethe efficiency of heat exchange in the heat source. This is because thetemperature difference at heat exchange in the heat source becomes largeas compared with the case where only the heat storage material 1 of thecentral portion 2 a is heated, since the heat transfer medium sent fromthe heat source is returned thereto with the temperature decreased.

A second procedure in the injection process of heat is adopted when thetemperature of the heat source is higher than that of the centralportion 2 a of the heat storage material 1, and at the same time, whenthe quantity of the heat of the heat source is all that is needed toheat the central portion 2 a. At this time, the heat transfer mediumfrom the heat source is sent to the heat exchanger 17 a, and heats theheat storage material 1 of the central portion 2 a, being then returnedto the heat source. Although the temperature of this heat transfermedium returned from the 17 a is higher than that of the heat storagematerial 1 of the outer portion 2 b, further heat transfer medium cannot be sent to the heat exchanger 17 b, unlike the case of the firstprocedure. This is because the quantity of heat of the heat source inthis case is not sufficient, and hence the heat transfer medium whichhas further decreased in temperature as a result of usage of the heatexchanger can not be heated by the heat source to a higher temperaturethan that of the heat storage material 1 of the central portion 2 a,with the result that a circulation wherein heat starts from the heatsource and returns thereto via the heat exchangers 17 a and 17 b can notthermally exist.

A third procedure in the injection process of heat is adopted when thetemperature of the heat source is lower than that of the central portion2 a of the heat storage material 1, and at the same time, when it ishigher than that of the heat storage material 1 of the outer portion 2b. In this case, the heat transfer medium is sent to the heat exchanger17 a, and heats the heat storage material 1 of the outer portion 2 b,being then returned to the heat source. Since the temperature of theouter portion 2 b lies between the temperature of the central portion 2a and the that of environment outside the heat storage device, it ispossible to cause the heat storage device to efficiently store theenergy of the heat source by using the heat exchanger 17 b alone, evenif the temperature of the heat source is not sufficiently high.

In the conservation process of heat, as in the case of the embodiment 1,the apparent viscosity of the heat storage material 1 of the outerportion 2 b is high so that the heat loss toward the outside is low.

Also in the extraction process of heat, the procedure varies inaccordance with the following three cases.

A first procedure is adopted when the temperature of the heat needed atthe outside is close to that of the heat storage material 1 of thecentral portion 2 a, and at the same time, when it can not be met byonly the heat of the central portion 2 a. At this time, the heattransfer medium exchanging heat with the outside is first sent from theheat source to the heat exchanger 17 b, and is preheated by the heatstorage material 1 of the outer portion 2 b, being then once returned tothe outside. Since the temperature of the heat transfer medium returnedfrom the heat exchanger 17 b is lower than that of the heat storagematerial 1 of the central portion 2 a, the heat transfer medium is nextsent to the heat exchanger 17 a, and is heated by the heat storagematerial 1 of the central portion 2 a, being then returned to theoutside. This makes it possible to rapidly extract the heat of the heatstorage material 1 of the outer portion 2 b and to suppress a rapidtemperature decrease of the central portion 2 a caused by a large loadfrom the outside.

A second procedure in the extraction process of heat is adopted when thetemperature of the heat needed at the outside is close to that of theheat storage material 1 of the central portion 2 a, and at the sametime, when it can be met by the quantity of heat extracted from thecentral portion 2 a. At this time, the heat transfer medium from theoutside is sent to the heat exchanger 17 a, and is heated by the heatstorage material 1 of the central portion 2 a, being then returned tothe outside for utilizing.

A third procedure in the extraction process of heat is adopted when thetemperature of the heat needed at the outside is lower than that of theheat storage material 1 of the central portion 2 a, but at the sametime, when it is higher than that of the heat storage material 1 of theouter portion 2 b. In this case, the heat transfer medium from theoutside is heated by the heat exchanger 17 b, and then it is returned tothe outside for utilizing. If the whole of the heat transfer medium issent from the outside to the heat exchanger 17 a, the temperature of theheat storage material 1 of the central portion 2 a having a highertemperature than the outer portion 2 b is reduced, which causes adecrease in the temperature of the heat storage material 1 existing inthe central portion 2 a having a temperature higher than that of theouter portion 2 b. This is undesirable because the effective energy(exergy) of the heat storage device is rendered less, even if thequantity of heat to be extracted from the heat storage device to theoutside is the same.

A third procedure is to prevent the reduction in the effectiveness ofheat storage device as much as possible, by extracting the heat from asuitable portion of the heat storage material 1 corresponding to thetemperature of the heat required by the outside. This makes it possibleto provide a heat storage device that efficiently operates in spite of awide variation in supply and demand of heat. In this embodiment, a casewhere the heat storage tank and the like are similar to those in theembodiment 1 is shown, but similar operations to the foregoing can beachieved also in the case of such constructions as shown in theembodiments 2 through 8.

Incidentally, when the heat of a temperature lower than that of theenvironment outside heat storage device is stored in the storage deviceof each embodiment, only the high-lower relation of temperature betweenthe storage device and the external environment, and the direction ofheat flow become opposite to those of each of the above-describedembodiments. However, the basic operation is the same as a correspondingembodiment.

The shape of the heat storage tank 2 in these embodiments are notlimited to the cuboid shape as shown in the drawings. Various shapessuch as cylindrical, or spherical shape may also be adopted.

What is claimed is:
 1. A heat storage device comprising: a heat storagetank having a central portion and an outer portion and charged with aheat storage material for storing heat supplied from the outside; heatexchange means for executing an injection and an extraction of heatbetween the inside of the heat storage tank and the outside by heatexchange between said heat storage material and a heat transfer medium,said heat exchange means being disposed so as to execute the heatexchange between the central portion of said heat storage tank and theoutside of the heat storage device; and suppressing means forsuppressing the natural convection of the heat storage material, saidsuppressing means being disposed in the outer portion of the heatstorage tank.
 2. A heat storage device as claimed in claim 1, whereinsaid suppressing means comprises a liquid-absorbent material dispersedin one of the heat storage material and the heat transfer medium in theouter portion in the heat storage tank.
 3. A heat storage devicecomprising: a heat storage tank having a central portion and an outerportion and charged with a heat storage material for storing heatsupplied from the outside; heat exchange means for executing aninjection and an extraction of heat between the inside of the heatstorage tank and the outside by heat exchange between said heat storagematerial and a heat transfer medium, said heat exchange means beingdisposed so that the central portion and the outer portion of said heatstorage tank are caused to sequentially perform a heat exchange with theoutside of the heat storage device; and suppressing means forsuppressing the natural convection of the heat storage material, saidsuppressing means being disposed in the outer portion of the heatstorage tank.
 4. A heat storage device as claimed in claim 3, whereinsaid suppressing means comprises a liquid-absorbent material dispersedin one of the heat storage material and the heat transfer medium in theouter portion in the heat storage tank.